The present invention relates to a dimming circuit for a high intensity discharge (HID) lamp. Circuits for dimming an HID lamp are known in the art.
One conventional circuit is shown in FIG. 1 and consists of an autotransformer, a power factor correction capacitor 2, a reactor ballast 3, a solid state phase control circuit module 4, a dimming control 5 and a lamp 6.
In the prior art circuit shown, the autotransformer converts the line voltage to the appropriate voltage to allow the reactor to act as a voltage limited current source for the lamp. The dimming control adjusts the phase angle of the current through the control module which controls the current to the lamp.
The disadvantage of this type of control is that at lower than normal current levels, depending upon the characteristics of the particular lamp, the arc in the lamp becomes highly unstable and extinguishes. Limited lamp dimming results with attendant susceptibility to variations in line voltage, ballast production tolerances and lamp tolerances.
The object of the present invention is to overcome the disadvantages of such conventional circuits.